Ask HN: How do you know when to *stop* working on something?

1) Have a goal

2) Are you still moving towards that goal? Or away from that goal?

The goal can be anything. For me, sometimes it's just "have fun building something". Am I still having fun? Keep building it. Not having fun? Move on. If my goal is "learn a new technology", then it's equally straight forward: "am I still learning?" or "did I learn everything I needed?"

Startups are the same: have a goal -- some objective, even if abstract, to guide you. You may change strategies, change products, change approaches along the way. But if you find that you are no longer moving towards that goal or no longer interested in moving closer to that goal, that's the end.

When you run out of the will or resources to continue, and when you have run out of ideas to make the situation better.